Add check for empty input to change_collation

[sijie123]

Description

Currently, if an empty input is provided to change_collation, the behaviour is as if no user input were made.
This causes a bug in db_operations.php, where a blank form will be returned to the user instead of an error message.

By checking for empty($_REQUEST['db_collation']) separately, we can catch bad user input and return the appropriate error message.

Fixes #14987

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For reviewer:
The github diff is misaligned. I essentially only added lines 86-88 and 136-144

[codecov]

Codecov Report

Merging #15031 into QA_4_8 will decrease coverage by 2.68%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             QA_4_8   #15031      +/-   ##
============================================
- Coverage     55.68%   52.99%   -2.69%     
  Complexity    14452    14452              
============================================
  Files           494      494              
  Lines         70878    64112    -6766     
============================================
- Hits          39467    33976    -5491     
+ Misses        31411    30136    -1275

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (QA_4_8@d198598). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             QA_4_8   #15031   +/-   ##
=========================================
  Coverage          ?   55.56%           
  Complexity        ?    14498           
=========================================
  Files             ?      494           
  Lines             ?    71071           
  Branches          ?        0           
=========================================
  Hits              ?    39494           
  Misses            ?    31577           
  Partials          ?        0

[williamdes]

@sijie123 Your PR works fine, can you implement the same check for tables ?

[williamdes]

Thanks for your initial contribution @sijie123 !